DS-CDMA integration spreading coherent receiver

ABSTRACT

The Spread Spectrum integration coherent receiver of the invention employs a time division multiplexing correlator bank and thus obtained coherent channel evaluator as a core. The receiver can execute initial PN synchronization, RAKE diversity coherent combination, AFC and adjacent cell search and combination receipt, and soft handoff. Further, “part capture in parallel based on slide energy window” and “tracking loop based on energy window barycenter” are introduced into the receiver of the invention, thereby simplifying the structure of the RAKE receiver. The receiver is capable of overcoming multipath fading, ensuring the RAKE receipt performance.

FIELD OF THE INVENTION

The present invention relates to a CDMA (code division multiple access)cellular communication system.

BACKGROUND OF THE INVENTION

Mobile communication techniques have become a widely used communicationmanner for their advantages such as flexibility and convenience since1980s. In a lot of mobile communication standards, CDMA cellularcommunication technique shows great potential for its featuresassociated with large capacity, simple frequency planning, goodcommunication quality and small electromagnetic interference. IS-95 CDMAcellular communication system proposed by Qualcomm Inc. and rapidlydeveloped all over the world uses this CDMA cellular communicationtechnique. Several candidate schemes of the third generation of digitalcellular communication system are established on the basis of CDMAtechniques.

Multipath fading which causes serious multipath interference exists in amobile communication system. In general, it is necessary to receivepilot signals with confirmation information so as to evaluate theamplitude and phase information of multipath signals, and it is possibleto achieve multipath diversity and coherent reception. A coherent spreadspectrum receiver which performs diversity process is referred to asRAKE coherent receiver. RAKE coherent receiver can correct phases of aplurality of singlepath signals which carry same information and areindependence from one another in fading features, and perform maximalcombination to overcome multipath fading and improve receivedsignal-to-interference ratio.

To achieve RAKE reception function, Synchronizing local spread spectrumsequence (PN code) with received signal is necessary. Thesynchronization is achieved by acquiring and tracking steps. Theacquiring step acquires a pilot channel and confirms that initialsynchronization (coars synchronization) of PN code is complete. Thecombination of these two steps provides PN code and accurate localtiming required for RAKE receiver.

Mobile communication spread spectrum receivers in CDMA cellularcommunication system have capabilities of diversity combination receiptto achieve soft handoff so as to improve receiving performance atboundaries of cells.

CDMA receivers have large local oscillation frequency-offset in turn-onlosing lock state due to the limit of cost. An automatic frequencycorrection (AFC) function is introduced into the receivers so that theRAKE receivers can operate normally in large local oscillationfrequency-offset state.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide directsequence spread spectrum integration coherent receiver designed with a“energy window barycenter” method to overcome the disadvantages due tothe non-determinacy of multipath signals in mobile communication. Thereceiver according to the invention can process multipath energy windowin parallel, and execute the operations such as synchronization track,RAKE diversity coherent combination, AFC and adjacent cell search andcombination receipt, and soft handoff, there by improving theperformance of CDMA spread spectrum receivers, and reducing hardware.

The present invention is implemented by the technical solution describedlater.

The Spread Spectrum integration coherent receiver according to theinvention uses a time division multiplexed correlator bank and thusobtained coherent channel evaluator as a core. Further, the receiver canexecute initial PN synchronization, RAKE diversity coherent combination,AFC and adjacent cell search and combination receipt, and soft handoff.Furthermore, “partially acquiring in parallel based on sliding energywindow” and “tracking loop based on energy window barycenter” areintroduced into the receiver according to this invention. Unlikeprocessing a singlepath as disclosed in patents issued to Qualcomm Inc.,the receiver according to the present invention can solve the problemscaused by multipath fading, ensure the RAKE receipt performance andsimplify the structure of the RAKE receiver.

According to an aspect of the invention, it provides a direct sequencespread spectrum/CDMA integration spread spectrum coherent receivercomprising a state control means for receiving control information froma CPU (center processor unit), generating control information used foroperating each of units, recording the generating unit for receiving aexternal clock, generating CPU interruption signals required for entiresystem, timing clock and time sequence by dividing frequency andcounting, and for adjusting the timing based on a PN code tracking unitand the state control unit; a correlator bank for performing effectivecorrelating integration by time division multiplexing one complexcorrelator to form equivalent correlators; a post correlation dataprocessing unit for receiving the output from one of correlator in thecorrelation bank, processing the data included in the output from thecorrelator, performing initial acquisition, adjacent cell search,selection of effective multipath based on the energy window according tothe control information from. state control unit; a PN code trackingunit for receiving a channel evaluation relating to effective multipathof pilot channel from the post correlation data processing unit,calculating a energy window barycenter and a loop filter to obtain amode value for a variable mode counter, and sending the mode value tothe timing generating unit to finely adjust the PN code generationclock, thereby adjusting local PN code phase; and an automatic frequencycorrection (AFC) loop calculating unit for performing frequency errorevaluation and loop filter calculation based on the informationassociated with effective multipath of the pilot channel, and sendingthe obtained result to a controllable frequency reference unit.

The principle of the invention will be discussed.

The algorithm executed by the CDMA spreading coherent receiver of theinvention includes parts of channel parameter evaluation, maximal ratiocombination, and associated initial PN acquisition, track, AFC, adjacentcell search, hand-off, and macro-diversity. Each of parts is describedas follows.

1. Channel Parameter Evaluation

A pilot channel in CDMA system is used for transferring a pilot sequenceknown in advance which may provide a system timing, extract carriers,evaluate channels, and execute hand-off, etc. The equivalent basebandreceiving signals may be expressed as shown in equation (1) when thesystem simultaneously transmits signals through a plurality of channels,

$\begin{matrix}{{r(t)} = {{\sum\limits_{n}{c_{n} \cdot {\sum\limits_{i}{s_{i}\left( {t - {n/W}} \right)}}}} + {z(t)}}} & (1)\end{matrix}$wherein s_(I)(t) represents the signs and equivalent baseband signalstransmitted through ith code division channel in downstream channels.The term of i=0 corresponds to the pilot channel. z(t) is complex WhiteGaussian noise of zero average value, c_(n) is a fading factor of nthpath of the channels. The purpose for evaluating channel parameter is toevaluate channel fading factor c_(n) based on the received signals r(t)and the known pilot sequence s₀(t).

It is assumed that frequency selectivity slow fading channel model isused as a mobile channel, c_(n) is then approximate to a constant withinthe channel evaluation region. The evaluation value of c_(n) is given asfollow:

$\begin{matrix}{\overset{\_}{c_{n}} = {{\frac{1}{{NE}_{c}}{\int_{0}^{{NT}_{c}}{{{r\left( {t - {nT}_{c}} \right)} \cdot {s_{0}^{*}(t)}}{\mathbb{d}t}}}} = {c_{n} + N_{a} + N_{c} + N_{z}}}} & (2)\end{matrix}$wherein N_(a), N_(c), and N_(z) are th outputs caused by multipathinterference, multiple access interference and white noise passedthrough a correlator due to the non-ideal correlation characteristic,T_(c) is a time width of one chip, NT_(c) is an integration region of achannel evaluation, and E_(c) is energy transmitted through a pilotchannel within one chip.2. Maximal Ratio Combination

After obtaining the channel parameter evaluation values for each ofpaths, other channels carrying data are coherently demodulated. To thisend, it only needs to despread each of paths of other channels. Thechannel evaluation parameter c_(n) obtained by using equation (2)weights the amplitude of despreaded results for each of paths andcorrects the phase thereof to make the despreaded results in phasecombine. This processing is referring to as maximal ratio combination.The equation will be described in detail as follows:

$\begin{matrix}{{\overset{\_}{d}}_{i} = {\frac{1}{E_{b}}{\sum\limits_{n}{\overset{\_}{c_{n}^{*}}{\int_{0}^{T_{s}}{{{r\left( {t - {nT}_{c}} \right)} \cdot {s_{i}^{*}(t)}}{\mathbb{d}t}}}}}}} & (3)\end{matrix}$wherein d _(i) is data transmitted on with channel carrying data, T_(s)is a sustaining interval of data, c _(n)* and c _(n) is a conjugatepair. In practice, it is not that all paths, which may be identified byRAKE receiver, have effective signal components. Comparing c _(n) with athreshold, only the multipath components which c _(n) are higher thanthe threshold are combined.3. Local Pilot Signal Restore

In foregoing channel evaluation, it is necessary to know pilot signalss₀(t). Therefore, the pilot signals s₀(t) are restored in local based onthe received signals r(t). Restoring the pilot signals s₀(t) comprisesacquiring step which performs coarsely synchronization (initialsynchronization) and tracking step which performs fine synchronization.Acquiring pilot signals is also referred to as PN codes acquisition.Tracking pilot signals is also referred to as PN codes track. In thisinvention, acquiring pilot signals is performed by using initialsynchronization method of CDMA cellular system based on the maximalenergy window. Tracking pilot signals is performed by using pilot signaltrack method based on multipath channel energy window barycentertracking loop.

Next, the principle of initial synchronization in CDMA cellular systembased on the maximal energy window is described.

In initial synchronization stage of a CDMA receiver, the phaseinformation of received signals can not be known. It is necessary toevaluate multipath fading channels in fraction intervals, and tryevaluation using local pilot sequence (PN code) with different phase. Inthis case, following equation (4) can be derived from equation (2).

$\begin{matrix}{{{{\overset{\_}{c}}_{n,m}(k)} = {\frac{1}{{NE}_{c}}{\int_{0}^{{NT}_{c}}{{{r\left( {t - {nT}_{c} - {{mT}_{c}/M}} \right)} \cdot {s_{0}^{*}\left( {t - {{kT}_{c}/M}} \right)}}{\mathbb{d}t}}}}},{m = 0},1,{{\Lambda\mspace{20mu} M} - 1}} & (4)\end{matrix}$wherein T_(c)/M is fraction sampling intervals, k is a possible certainphase parameter of the local pilot PN sequence.

The effective distribution range of channel fading factor c_(n) inequation (1) is defined as energy distribution window of multipathsignal (hereinafter is referred to as multipath energy window). The sizeof the window may be determined by time-delay extend range of multipathchannels. For the sake of simplifying discussion, the effectivedistribution range of c_(n) may be set to n∈[−L₁,L₂]. The size of thewindow in multipath fading circumstances may be set differently fordifferent areas, for example, 3 μs for cities, 6 μs for countries, and15 μs for mountain areas. The size of window is associated with thecircumstances where the cellular communication system is located, and isregardless of the used frequency band. The size of multipath energywindow may be selected according to the maximal likelihood value, forexample, no more than 30 μs, and then the value of L=L₂−L₁+1 is not morethan 30 μs/T_(c) so that a spread spectrum receiver can be used invarious circumstances.

In a multipath energy window, not all signal arrival paths areeffective. To this end, a threshold may be set to judge the signalenergy (i.e., intensity of c_(n)) for each of paths in the window. Asignal arrival path is judged as effective path when the signal energyis larger than the threshold. Otherwise, the path is judged as a pureinterference path. To avoid the degradation of the performance, thecalculation is not applied to all pure interference paths. The thresholdis set slightly larger than the side lobe value of a pilot signal (PNcode) partial correlation value.

To obtain sufficient acquisition precision, a receiver samples thereceived signals using over-sampling technique. The sampling rate is Mtimes the chip rate of PN code. Assuming the length of PN code requiredfor synchronizing is p, the PN code acquiring method of the inventionselects a phase from M×P possible PN code phases, and maximize themultipath energy contained in the multipath energy window.

According to above concept of multipath energy window, the multipathenergy window which the phase of local PN code is k is defined asfollow:

$\begin{matrix}{{E_{win}(k)} = {\sum\limits_{n = {- L_{1}}}^{L_{2}}{\sum\limits_{m = 0}^{M - 1}{{{\overset{\_}{c}}_{n,m}(k)}}^{2}}}} & (5)\end{matrix}$the acquiring method based on multipath energy window is there describedas the selection of a value k which makes following equation (6) have amaximal value from all possible values k of local PN code phase:

$\begin{matrix}{\max\limits_{k}\;{E_{win}(k)}} & (6)\end{matrix}$On the other hand, it can be seen from equation (4), the multipathenergy window calculation as shown in equation (5) exists followingderivative relationship associated with a sliding window:E _(wln)(k+1)=E _(wln)(k)−| c _(L) ₂ _(M−1)(k)|² +| c _(L) ₁ _(,0)(k+1)|²  (7)Thus, Initial synchronization calculation can be greatly simplified.

The method of searching adjacent cells is similar with the initialsynchronization method of PN code except for that the PN code utilizedin the equation is a pilot signal sequence in a certain adjacent cell,the region to be searched is a designating region in advance by a basestation, but not all possible phases of PN codes.

Next, the principle of pilot channel tracking method based on themultipath energy window barycenter tracking loop is described.

If K denotes the evaluated result of kth channel, the barycenter ofcorresponding multipath energy window is given bycg(k)=cg_(w)(k)/cg_(s)(k), wherein cg_(w)(k) and cg_(s)(k) arecalculated as follows:

$\begin{matrix}\begin{matrix}{{{{cg}_{w}(k)} = {\sum\limits_{n}{n{{\overset{\_}{c_{n}}(k)}}^{2}}}},} & {{{cg}_{s}(k)} = {\sum\limits_{n}{{\overset{\_}{c_{n}}(k)}}^{2}}}\end{matrix} & (8)\end{matrix}$wherein n corresponds to the position where the multipath fading factorc_(n) (k) locates in multipath energy window. It should be noted thateach of c_(n) (k) S to be calculated in equation (8) is an effectivesignal arrival path which is large than designated threshold.

PN code tracking loop for multipath energy window barycenter is designedsuch that the target position of a multipath energy window barycenter isset to cg_(target) so that the PN code phase of the receiver can beadjusted by detecting the difference between the multipath energy windowbarycenter r value cg(k) and cg_(target) to reduce the difference. Forsimplifying the calculation, it s assumed that cg_(target) is set tozero, the phase adjustment of local PN code can be then performed bysimply judging the polarity of cg_(w)(k), but not need to calculatecg_(s)(k) and cg(k).

To avoid incorrect adjustment due to the random changes of multipathfading signals and channel evaluation errors, the barycenter evaluatingvalue calculated by equation (3) is smoothly filtered. Assuming thesmoothly filtered evaluating value is cg_(w)(k), the adjusting operationcan generalized to:let the phase of local PN code lead δ if cg_(w)(k)>0let the phase of local PN code lag δ if cg_(w)(k)<0let the phase of local PN code hold if cg_(w)(k)=0  (9)

The local PN code phase adjusting unit performs the operation as shownin equation (9). According to an embodiment of the present inversion,the local PN code phase adjustment is executed by finely adjusting thetransmitting clock of local PN code. FIG. 2 illustrates an operationflowchart of the method according to the invention. In FIG. 2, the PNcode clock is generated by courting the frequency division of a multipletimes (M times) the external clock. A variable mode counter finelyadjusts the chip clock. The mode value of the counter is M−1 if thecg_(w)(k) is positive. The mode value is M+1 if the cg_(w)(k) isnegative. Otherwise, the mode value of the counter is M. In this way,the PN code phase can be adjusted as shown in equation (9), and thephase difference of fine adjustment is δ=T_(c)/M, wherein M may be 32 or64 to measure the adjustment accuracy enough.

4. Automatic Frequency Correction (AFC)

In practice, the stability of initial frequency in a mobile terminal islimited to about 1 ppm because of the restrict by volume and cost, etc.This results in there are approximate several hundred Hz to several KHzfrequency difference between a base station and a mobile terminal.Therefore, it is necessary to introduce an automatic frequencycorrection (AFC) function into mobile terminals, thereby preventing thedegradation of the system performance. In view of the effects due to thefrequency difference between transmitting side and receiving side, theequivalent baseband model in equation (1) can be depicted as equation(10):

$\begin{matrix}{{r(t)} = {{\sum\limits_{n}{{c_{n} \cdot {\mathbb{e}}^{j\;\Delta\;\omega_{c}t}}{\sum\limits_{l}{s_{i}\left( {t - {n/W}} \right)}}}} + {z(t)}}} & (10)\end{matrix}$wherein Δω_(c) is the frequency difference between a transmitting sideand a receiving side. The channel evaluation as shown in equation (2)can be modified as shown in equation (11):

$\begin{matrix}\begin{matrix}{\overset{\_}{c_{n}} = {\frac{1}{{NE}_{c}}{\int_{0}^{{NT}_{c}}{{{r\left( {t - {nT}_{c}} \right)} \cdot {s_{0}^{*}(t)}}{\mathbb{d}t}}}}} \\{= {{c_{n} \cdot \left\{ {{\mathbb{e}}^{j\;\Delta\;\omega_{c}{{NT}_{c}/2}}\frac{\sin\left( {\Delta\;\omega_{c}{{NT}_{c}/2}} \right)}{\Delta\;\omega_{c}{{NT}_{c}/2}}} \right\}} + N_{a} + N_{c} + N_{z}}} \\{\cong {{c_{n}{\mathbb{e}}^{j\;\Delta\;\omega_{c}{{NT}_{c}/2}}} + N_{a} + N_{c} + N_{z}}}\end{matrix} & (11)\end{matrix}$wherein assuming Δω_(c)NT_(c)/2<<1. The evaluation value of Δω_(c) isobtained by using the evaluation value of c _(n) in two sequentialregions t∈[0, NT_(c)] and t∈[(N+1)T_(c),(2N+1)T_(c)], and assuming thatc_(n) does not charge in the two sequential regions. The localoscillator source of a mobile terminal can be adjusted by using theobtained evaluation, thereby achieving AFC function.5. Soft Hand-off and Macro-diversity

Soft hand-off and macro-diversity are essential function for a CDMAcellular communication system. A mobile terminal detects the intensityof signals from adjacent base stations when the mobile terminal entersboundaries of two or more adjacent cells. When the intensity of thesignal from a certain base station is larger than a predeterminingvalue, the mobile terminal enters into macro-diversity state,communicates with two or more base stations simultaneously, and combinesthe same data transmitted from the two or more base stations to improvethe performance of the mobile terminal when it is in boundaries ofcells.

The detection of signal intensity, which is required by soft hand-off,from adjacent base stations can be achieved by evaluating the intensityof pilot channel transmitted from the adjacent base stations. This canbe accomplished by replacing the pilot signals in equation (2) with thepilot signals from the adjacent base stations and performing channelevaluation on a certain multipath distributing region. When theevaluated pilot signals are larger than a predetermined intensity, themobile terminal informs the base station with which is communicating ofthe event and prepares to enter into a macro-diversity state.

The signals from a plurality of base stations need to be received andcombined when the mobile terminal enters into a macro-diversity state.This can be accomplished by replacing the spread spectrum signals(sequence) in equation (3) with the spread spectrum signals transmittedfrom the adjacent base stations and simultaneously receiving data fromtwo or more base stations in macro-diversity state, and then performingpost combination after aligning in time

THE ADVANTAGES OF THE INVENTION

The present invention provides a initial synchronization method based onthe energy window and PN code track method based on the energy windowbarycenter with respect to the random change characteristic in multipathfading circumstances. This method does not need to individually processeach delay path. Therefore, the stability of a spread spectrum receiverin multipath fading circumstances is improved. This invention alsoinduce the operation which need to be performed by a spread spectrumreceiver into equations (2) (or (5)) and equation (3). Further, thisinvention provides a design method used for spread spectrum receivers sothat the hardware used in spread spectrum receivers is greatly reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description withreference to the accompanying drawings, which illustrate examples of thepresent invention.

FIG. 1 schematically illustrates a configuration of a spread spectrumreceiver according to an embodiment of the invention; and

FIG. 2 schematically illustrates a diagram showing the PN code phaseadjustment according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the accompanying drawings, there are shown preferredembodiments of the invention. As described above, each of parts in aspread spectrum receiver is based upon equations 3 and 11 (or equation2). The spread spectrum receiver can accomplish functions such asinitial synchronization of the pilot channel, track and channelevaluation, multipath maximal ratio combination, AFC, adjacent cellsearch, hand-off, and macro-diversity. Accordingly, this inventionprovides the arrangement of a CDMA spread spectrum receiver.

A preferred embodiment of the invention will be discussed with referenceto FIG. 1.

The CDMA spread spectrum receiver according to the invention comprises astate control unit (FSM_CONTROL), a timing generating unit (SYS_CLK), adata delay line unit (DELAY_LINE), a correlator bank (CORRELATOR_BANK),a post correlation data processing unit (POST_CPRR), a RAKE combiningunit (RAKE_COMB), a post combining unit (POST_COMB), a PN codegenerating and sliding unit (PN_GROUP), a WALSH function generating unit(WALSH_GEN), an AFC loop calculating unit (AFC_LOOP), and a PN codetracking unit. The functions and operation of each of the units aredescribed as follows.

1. State Control Unit (FSM_CONTROL)

The state control unit includes a CPU interface block, A receiver statetransformation control (TOP_FSM) block, and a unit state control signalgenerating (DOWN_FSM) block. The state control unit interacts with theCPU, which is controlled by baseband, of information, receives controlinformation from the CPU, generates control information used foroperating state transformation of each of units, records the operationstates of each of the units and reports the states to the CPU. The CPUinterface has a build-in RAM. The storage capacity of the RAM may bedetermined as desired, for example, 64×8 bits. The CPU read/writes theRAM in the manner of interrupting poll, interacts with the statetransformation control block of the receiver to transfer information.TOP_FSM scans the RAM in the period of interrupting signals (e.g., 26.27ms, 20 ms, or 10 ms depended upon the application or the state of areceiver) and receives control information from the CPU (e.g., acquiringstate, searching state, receiving states of various code channel) andsystem parameters (e.g., the number of code channel, spread spectrumrate, searching region, and frame-offset, etc.) required for arrangingoperation so as to determine next operation state. Further, the TOP_FSMinstructs the DOWN_FSM to generate control signals for other units. Theinformation indicating that state transformation is complete istransferred to the RAM unit so that the CPU can obtain correspondingfeedback information.

2. Timing Generating Unit (SYS_CLK)

The timing generating unit receives external clocks (in general, 16times or 32 times the spread spectrum sequence chip rate), generates CPUinterruption signals required for entire system, timing clock and timesequence by dividing frequency and counting, and adjusts the timingbased on a PN code tracking unit and the state control unit. The timinggenerating unit generates N sets of timing signals dependent on thereceiving links of each of base stations to support macro-diversity ofat most N base stations. Further, The timing generating unit tracks thetiming in accordance with the channel evaluation of respective basestations and the result of the tracking unit. In the process ofmacro-diversity, the relative time-delay changes of respective receivinglinks are measured and reported to the CPU. The CPU controls thepost-control unit to perform macro-diversity function after aligning thetime-delay of respective links.

3. Data Delay Line Unit (DELAY_LINE)

The data delay line unit comprises four sets of RAMs, which storagecapacity is, for example, 18×6 bits, or D flip-flops. The data delayline unit samples 4 times the input data and outputs 72 delay taps with¼ chip interval. The output data is provides to a correlator bank unit.

4. Correlator Bank Unit

The correlator bank comprises four banks of correlators. Each bank ofcorrelators perform effective correlation integration 31 times by timedivision multiplexing one complex correlator (multiplexing with 32 timesthe chip rate), thereby forming 31×4 equivalent correlators sum total.Each of equivalent correlators performs the calculation as shown inequations 3 and 5 (or 2) with the chip interval. Correlators in eachbank are numbered 0 to 30 in accordance with the sequence of timedivision multiplexing. Correlators 0-17 in each bank (sum to 4×18equivalent correlators in four banks) are used for evaluating multipathchannel in parallel. A following POST_CORR unit performs acquisition,adjacent cell search and effective selection etc. on the evaluatedresults. Correlators 8-30 in each bank are used for evaluating channelsof effective paths from base stations and despread data carried on datachannel. Marco-diversities of three base stations are supported.

The configuration of a correlator bank may be arranged as desired so asto easily support different system standard.

5. Post Correlation Data Processing Unit (POST_CORR)

The post correlation data processing unit receives the output from thecorrelators in correlation banks, and processes the data included in theoutput from the correlators, performs initial acquisition, adjacent cellsearch, selection of effective multipath based on the energy windowaccording to the control signals from FSM_CONTROL unit. The processedresults are provided to the PN code tracking unit, AFC loop unit and thestate control unit.

6. RAKE Combining Unit (RAKE_COMB)

The RAKE combining unit receives the channel evaluated results fromPOST_CORR unit and decorrelates the data stream, and combines (equation3) the effective multipath according to the control signals fromFSM_CONTROL unit. The results are provided to a post combining unit.

7. Post Combining Unit (POST_COMB)

The post combining unit receives multipath combining results fromRAKE_COMB unit, and determines whether or not to perform macro-diversityof a plurality of base stations according to the control signals fromFSM_CONTROL unit. If macro-diversity is necessary, the post combiningunit delays paths based on the time-delay difference of respective basestations provided by the CPU so as to align the paths to each other intime, and performs macro-diversity of a plurality of base stations.

8. PN Code Generating and Sliding Control Unit (PN_GROUP)

Five groups of PN codes are provided, wherein three of them are used forthe despread data of three base stations. One of rest groups of PN codesis used for adjacent cell, the other group of PN codes is used fortransmitter. The PN code used for adjacent cell search depends on mainreceiving links. The instantaneous process in PN code sliding process isshield to avoid confusing the demodulation results of the receiver. Thetiming of PN code used for transmitters depends on the timing of basestations which have been acquired when a receiver is turned on. When alink is released, the PN code of the link should synchronize with the PNcode of main receiving link so that the relative reference positionsamong PN codes are in known state.

9. Walsh Function Generating Unit (WALSH_GEN)

The Walsh function generating unit is controlled by the slate controlunit (FSM_CONTROL) and the timing generating unit (SYS_CLK), andgenerates three Walsh sequences depended on respective links.

10. AFC Loop Calculating Unit (AFC_LOOP)

The AFC loop calculating unit evaluates frequency error and calculatesloop filter based on the effective multipath information of pilotchannel provided from FSM_CONTROL and SYS_CLK, and send the result to acontrollable frequency reference unit.

11. PN Code Tracking Unit (CG_LOOP TRACKING)

The PN code tracking unit receives the effective multipath channelevaluation of pilot channel from the post correlation data processingunit, calculates the energy window barycenter and loop filter, andobtains a mode value of a variable mode counter. The result is sent tothe SYS_CLK unit to finely adjust the timing of local PN code, therebyadjusting the phase of local PN code.

Next, the operation of main function of CDMA spread spectrum receiverwill be described.

1. Initial Acquisition Function

The CPU writes initial acquisition state control word into FSM_CONTROLunit. The control word has initial acquisition control command, thelength of search region and the number of PN code used for slidingcorrelation, and integrating periods etc. When the start position ofnext frame is arrived at, the TOP_FSM block receives the initialacquisition information through the interface. Then, the TOP_FSM blockinitializes the acquisition, informs the DOWN_FSM block of thegeneration of PN code state control signal, the integrating periodcontrol signal of the CORRELATOR_BANK unit and the POST_CORR unitacquisition state control word etc.

The PN_GROUP unit periodically slides PN code after receiving the numberof the used PN code and the number of sliding chips every time. Theoutput of the PN_GROUP unit jumps 16 chips every integrating period andsends to the CORRELATOR_BANK unit.

The CORRELATOR_BANK unit receives baseband sampling input signals and PNcode signals as described above. Then, the CORRELATOR_BANK unitperiodically performs correlation calculation as shown in equation 2 or5 based on the control signals from the DOWN_FSM block. Forty-sixmultipath channel evaluations with ¼ chip interval are obtained, and theresults are sent to the POST_CORR unit for followed process.

The POST_CORR unit receives the parallel integration output from theCORRELATOR_BANK unit, and calculates the sliding energy window andcompares it with a maximal value according to the control signals fromthe DOWN_FSM block.

Repeating above processes, the DOWN_FSM block send acquisition stopsignal when the length of search period designated by the CPU is over.The POST_CORR unit sends the position and energy value of the maximalsliding energy window to the FSM_CONTROL unit and then read by the CPU.

The CPU obtains the position and energy value of the maximal slidingenergy window and determines whether the energy is larger than the basicenergy required for acquisition. If it is positive, the CPU sends theinformation of sliding PN code to the FSM_CONTROL unit. The FSM_CONTROLunit controls the corresponding PN code to establish the requiredinitial synchronization PN code (hereinafter referring to as mainsynchronization code). If it is negative, this acquisition is fail.

After finishing initial synchronization, the CPU immediately informs theFSM_CONTROL unit to enter synchronization tracking state fit this time,the CORRELATOR_BANK unit performs correlation calculation based on theestablished main synchronization code. The result is sent to thePOST_CORR unit. The POST_CORR unit selects effective paths, calculatesthe barycenter position, and generates a PN fine adjustment signalsbased on the shift of the barycenter. The SYS_CLK unit finely adjustschip timing based on the fine adjustment signal to keep thesynchronization of chip timing. Also, the CPU informs the FSM_CONTROLunit to perform AFC operation, and the result is used for adjusting themain reference clock of the RF block in the receiver.

2. Data Despread Function

When the receiver despreads a certain code channel, the CPU writes statecontrol signals and parameters including the number of a code channel(WALSH sequence number), Integrating length etc. into the FSM_CONTROLunit. The TOP_FSM block reads the information when an interruptionarrives at after the FSM_CONTROL unit receives the information from theCPU and informs the DWON_FSM block to generate associated controlsignals.

The PN_GROUP unit provides main PN code used for despread data.

The WALSH_GEN unit generates associated WALSH sequence number.

The CORRELATOR_BANK unit receives baseband sampling signals, mainsynchronization PN code and WALSH sequence, and performs integratingoperation as shown in equation 3 based on the integration period controlsignal generated by the DWON_FSM block. At the same time, theCORRELATOR_BANK unit perform channel evaluating operation as shown inequation 5 (performing the evaluation of 4×18 channel parameters everyintegration interval). The POST_CORR unit extracts the result and sendsit to the RAKE_COMB unit.

The function operation for despreading data, which is executed by thePOST_CORR unit, relates to two aspects. On one hand, the number andposition of the effective multipath is determined according to thereceived pilot signal, and then sent to the CORRELATOR_BANK unit so asto determine the position of despreading data associated with effectivemultipath in next integration interval. On the other hand, the channelparameters of effective multipath are chosen and then sent to RAKE_COMBunit to perform multipath combination.

The RAKE_COMB unit receives the results of effective multipath parameterevaluation and data despread to combine the maximal ratio, and send thecombining result to a channel decode unit through a parallel interface.

3. Adjacent Cell Search Function

The process of adjacent cell search function is similar with the processof initial acquisition except for that the adjacent cell search functionneed to be performed along with other functions (for example, datadespread function) simultaneously. The regions to be searched are thelocal areas designated by CPU.

4. Macro-Diversity and Soft Hand-off Function

The accomplishment of macro-diversity and soft Hand-off is more complexthan other functions, which includes a macro-diversity preparing stage,a macro-diversity implementing stage, and a macro-diversity removingstage. In the macro-diversity preparing stage, the operation includes:

-   -   a. A mobile station searches the pilot signal intensity of each        of base stations in accordance with the requirements of the base        stations during the mobile station communicates with a single        base station. When the signal intensity of station reports the        base station of the searching result. After receiving a response        from the base station, the mobile station modifies an active set        maintained in the mobile station.    -   b. The time-delay from each base station to the mobile station        is calculated for the purpose of determining the relationship        between sign and time-delay of each base station. The calculated        time-delay is reported to the CPU and provided to the POST_COMB        unit to align the arrival time-delay combined by each base        station

After finishing the macro-diversity preparing stage, the mobile stationenters the macro-diversity implementing stage. The mobile stationsearches the changes of signal intensity and the time-delay arrived atthe mobile station for each of the base stations in real time while itcombines a plurality of arrival signals of base stations. The mobilestation adjusts the signs and delays of the signals arrived at themobile station from each of base stations to ensure receiversynchronously receives signals from a plurality of base stations whilethe mobile station measures the intensity of pilot signal from each ofbase stations.

A T_Drops timer is started when the intensity of pilot signal from acertain base station is lower than the threshold. If the timer isexpired, the processing proceeds to the macro-diversity removing stage.

In macro-diversity removing stage, the mobile station resets all timingsand counts associated with the base stations which macro-diversity is tobe removed. The PN code timing used in macro-diversity is restored tothe state of synchronizing with a main base station. The pilot signalused by the base station is removed from the active set.

EXAMPLE

Next, the implement of the present invention is described with a mobileterminal in CDMA 2000 system used as an example. The mobile terminal maybe a vehicle mobile station in CDMA 2000 cellular mobile communicationsystem fitting Standard 3GPP2 Release A. The spread spectrum receivingpart in the mobile station can be implemented by, for example, aXC4085xla FPGA chip, a product of Xilinx company. The main parametersare listed as follows:

Spreading chip rate is 1.2288 MHz;

I/Q sampling rate is 4×1.2288 MHz, 6 bits input;

External clock (EXT_CLK) is 39.3219 MHz;

Integrating period for channel evaluating is 384 chip intervals (N=384);

Initial synchronization time is 0.75 s;

Applicable range of AFC is ±2 KHz;

Data transformation rate is 19.2 kbps to 307.2 kbps.

The spread spectrum receiver according to the invention can provideexcellent stability in the circumstances of vehicle mobile terminals.

The spread spectrum receiver according to the invention performs theoperation as shown in equations (2) (or (5)) and equation (3) employs atime division multiplexing correlator bank and thus obtained coherentchannel evaluator as a core. Further, The receiver includes a statecontrol unit (FSM_CONTROL) and a post correlation process unit.Furthermore, the receiver can execute initial PN synchronization, RAKEdiversity coherent combination, AFC and adjacent cell search andcombination receipt, and soft handoff,

INDUSTRY PRACTICABILITY

1) The spreading coherent receiver according to the invention uses CDMAcellular system initial synchronization method based on a maximal energywindow such that the RAKE receiver operates in the maximal energy windowand improves the stability of acquiring initial synchronization

2) This invention uses a method for pilot channel tracking based onmultipath channel barycenter tracking loop. The receiver according tothe invention tracks multipath energy window, but not every delay path.Therefore, the stability of a spread spectrum receiver in multipathfading circumstances is improved and the hardware used in a spreadspectrum receiver is greatly reduced. For tracking N base stations, itis only necessary to establish N energy barycenter tracking loops.

3) The spreading coherent receiver according to the invention uses timedivision multiplexed correlator banks to search, thereby greatlyincreasing the search speed.

4) The spreading coherent receiver according to the invention canaccomplish functions such as initial synchronization of the pilotchannel, track and channel evaluation, multipath maximal ratiocombination, AFC, cell search, hand-off, and macro-diversity.

5) The design of the spreading coherent receiver according to theinvention can be described by using VHDL or Verilog languages,implemented by FPGA or ASIC. DSP core or external DSP chip is notnecessary in the spreading coherent receiver according to the invention.

Although embodiments of the present invention have been shown anddescribed, it will be understood by those skilled in the art thatchanges may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe appended claims and their equivalents.

1. A direct spread spectrum/CDMA integration spreading spectrum coherentreceiver comprising: a state control means for receiving controlinformation from a CPU (center processor unit), generating PN code statecontrol signals to be inputted into a PN code generating and slidingcontrol unit, integrating period control signals to be inputted into acorrelator bank, acquisition state control word to be inputted into apost correlation data processing unit, and state control word to beinputted into a post combing unit; a timing generating unit forreceiving a external clock, generating CPU interruption signals requiredfor entire system, timing clock and time sequence by dividing frequencyand counting, and for adjusting the timing based on a PN code trackingunit; the correlator bank for receiving input signals from a data delayline unit, PN code signals from the PN code generating and slidingcontrol unit, Walsh code signals from a Walsh function generating unitin order to perform the complex correlation integration between theinput signal and the PN code signal multiplied by the Walsh codingsignal, integrating period control signals form the timing generatingunit respectively, and for periodically performing effective correlatingintegration by time division multiplexing one complex correlator to formequivalent correlators; the post correlation data processing unit forreceiving the output from one of the correlators in the correlator bank,processing the data included in the output from the correlator,performing initial acquisition, adjacent cell search, selection ofeffective multipath based on the energy window according to the controlsignals from the state control unit; the PN code tracking unit forreceiving a channel evaluation relating to effective multipath of pilotchannel from the post correlation data processing unit, calculating thebarycenter of a energy window and a loop filter to obtain a mode valuefor a variable mode counter, and sending the mode value to the timinggenerating unit to finely adjust the PN code generating clock, therebyadjusting local PN code phase; and an automatic frequency correction(AFC) loop calculating unit for performing frequency error evaluationand loop filter calculation based on the information associated witheffective multipath of the pilot channel from the post correlation dataprocessing unit, and sending the obtained result to a controllablefrequency reference unit.
 2. The direct spread spectrum/CDMA integrationspreading spectrum coherent receiver according to claim 1, wherein thetiming generating unit receives external clocks which are 16 times or 32times the spread spectrum sequence chip rate.
 3. The direct spreadspectrum/CDMA integration spreading spectrum coherent receiver accordingto claim 1, wherein the correlator bank comprises four banks ofcorrelators, the correlators in each bank perform effective correlationintegration 31 times by time division multiplexing one complexcorrelator with 32 times the chip rate, thereby forming 31×4 equivalentcorrelators.
 4. The direct spread spectrum/CDMA integration spreadingspectrum coherent receiver according to claim 1, further comprising adata delay line including random access memory (RAM) or D flip-flop forfour times sampling input data.
 5. The direct spread spectrum/CDMAintegration spreading spectrum coherent receiver according to claim 4,wherein the data delay line unit comprises four sets of RAM, whichstorage capacity is 18×6 bits, or D flip-flops, for outputting 72 delaytaps with ¼ chip interval, and sending output data to the correlatorbank unit.
 6. The direct spread spectrum/CDMA integration spreadingspectrum coherent receiver according to claim 1, further comprising aRAKE combining unit for receiving the channel evaluated results from thepost correlation data processing unit and decorrelating the data stream,and combining the effective multipath according to the control signalsfrom the state control unit.
 7. The direct spread spectrum/CDMAintegration spreading spectrum coherent receiver according to claim 1,in further comprising a post combining unit for receiving multipathcombining results from RAKE combining unit, and determining whether ornot to perform macro-diversity of a plurality of base stations accordingto the control signals from state control unit, if macro-diversity isnecessary, the post-combining unit delays paths based on the time-delaydifference of respective base stations provided by the CPU to align thepaths to each other in time, and performs macro-diversity of a pluralityof base stations.
 8. The direct spread spectrum/CDMA integrationspreading spectrum coherent receiver according to claim 1, furthercomprising a PN code generating and sliding unit for providing fivegroups of PN codes.
 9. The direct spread spectrum/CDMA integrationspreading spectrum coherent receiver according to claim 8, wherein threegroups of PN codes provided by the PN code generating and sliding unitare used for despreading data of three base stations, one group of PNcodes are used for searching adjacent cell, and the other group of PNcodes are used for transmitter.
 10. The direct spread spectrum/CDMAintegration spreading spectrum coherent receiver according to claim 1 ,further comprising a Walsh function generating unit which is controlledby the state control unit and the timing generating unit for generatingWalsh sequences depending on respective links and providing generatedWalsh sequences to the correlator bank unit.
 11. The direct spreadspectrum/CDMA integration spreading spectrum coherent receiver accordingto claim 7, wherein the macro-diversity comprises a macro-diversitypreparing stage, a macro-diversity implementing stage, and amacro-diversity removing stage.